1. Field of the Invention
The present invention relates to a mirror positioning structure for correcting skew and bow in a laser scanning unit and a laser scanning unit employing the same. More particularly, the present invention relates to a mirror positioning structure of a laser scanning unit and a laser scanning unit employing the same, in which a reflection mirror for correcting skew or bow can be easily rotated or moved in a straight line.
2. Description of the Related Art
In general, a laser printer is a printing device which scans laser light emitted from a laser diode onto a photoconductor to form a latent image, and transfers the latent image formed on the photoconductor onto a printing medium such as paper to produce a visual image. A laser scanning unit (LSU) is an image forming device which generates laser light in such a laser printer to form a latent image on a photoconductor. FIG. 1 is a schematic view of a conventional LSU 100. Referring to FIG. 1, the LSU 100 includes a light source 111, a collimating lens 112, an aperture 113, a cylindrical lens 114, a beam deflection mirror 115, a scanning lens 116, and a photoconductive drum 118. The light source 111, the collimating lens 112, the aperture 113, the cylindrical lens 225, the beam deflection mirror 115, and the scanning lens 116 are installed in a housing to protect them from dust generated inside the printer.
In such a configuration, a light beam emitted from the light source 111, such as a laser diode, is converted into a parallel beam. The parallel beam is restricted by the aperture 113, and passes through the cylindrical lens 114 to be converged into a linear light beam that extends horizontally in a sub-scanning direction. Then, the linear light beam is moved with a uniform speed in a main scanning direction, that is, the horizontal direction of the paper, by the beam deflection mirror 115, which rotates and passes through the scanning lens 116 and is reflected by a reflection mirror 117 (see FIG. 4) onto the photoconductive drum 118. The beam deflection mirror 115 can be a polygon mirror, for example. Further, the scanning lens 116 has a uniform refractive index with respect to the axis of the light and refracts the light beam reflected from the beam deflection mirror 115 at a uniform speed along a main scanning direction to focus the light on the photoconductive drum 118.
The light beam which passes through the scanning lens 116 in the LSU 100 must be scanned in a straight line along the main scanning direction onto a surface to be scanned, such as the photoconductive drum 118. However, referring to FIG. 2, the light beam deviates minutely in the sub-scanning direction due to assembly tolerances or other aberrations. Thus, the light beam is not scanned in a straight line in main scanning direction on the surface to be scanned. When the ends A and B of the scanning line formed by the beam spot deviate along the sub-scanning direction, the scanning path is skewed. When the scanning line is not straight but instead is bent, the scanning path is bowed.
Distortions in the scanning line such as skew or bow decrease the printing precision and image quality. In a tandem laser scanning unit used in a color laser printer, skew or bow are major problems. In a color laser printer using a tandem laser scanning unit, photoconductive drums for colors such as magenta, yellow, cyan, and black are separately installed, and when different distortions of scanning lines occur on the photoconductive drums, the quality of the produced color images deteriorates.
Conventionally, scanning line distortion is corrected by directly transforming the scanning lens or the reflection mirror, or by placing a reflection mirror between the scanning lens and the photoconductive drum to adjust the angle of the laser beam.
FIG. 3 illustrates a conventional correction apparatus for correcting scanning line distortion. The conventional correction apparatus of FIG. 3 fixes the scanning lens 116 to the housing 110 using two supporting members 131, and presses the scanning lens 116 using a screw 130 which is movably installed on the housing 110. Also, protrusions 131a are formed at an end of each of the supporting members 131. The conventional correction apparatus of FIG. 3 presses the scanning lens 116 using the screw 130 and the supporting members 131 and transforms the scanning lens 116 to correct the scanning line.
FIG. 4 illustrates a conventional structure in which scanning line distortion is corrected by adjusting the inclination of the reflection mirror 117. Referring to FIG. 4, a groove 125 is formed in the housing 110, and the reflection mirror 117 is inserted in the groove 125 so that it is inclined. A screw 122 is installed in a position on the frame 110 corresponding to the upper portion of the reflection mirror 117. A fixing spring 120 and a screw 121 fix the reflection mirror 117 in the groove 125. In such a configuration, the angle of the reflection mirror 117 is adjusted by rotating the screw 122 to adjust the insertion depth of the screw 122.
However, when the scanning lens or the reflection mirror is adjusted by force using a screw, secondary distortion can occur since stress remains in the scanning lens 116 or in the reflection mirror 117. Further, to effectively correct the scanning line distortion, not only the rotation angle of the reflection mirror between the scanning lens 116 and the photoconductive drum 118 need to be adjusted, but the incident location of the beam on the scanning lens 116 needs to be adjusted to correct the incident location of the laser beam on the scanning lens.
Accordingly, a new technique is needed to correct the scanning line distortion by adjusting the incident angle and location of the laser beam at the same time.